This invention relates to a method for preparing a highly heat-conductive substrate and a copper wiring sheet (hereinafter "Cu wiring sheet") usable in the method.
Recently for a power semiconductor element, various requirements such as high density integration, hybridization and further control of a large quantity of current and the like made severe. When an attempt is made to satisfy the aforesaid requirements, a large amount of heat generated from the semiconductor element raises a problem. Thus, it is necessary to radiate the large quantity of heat generated to prevent elevation of temperature of the semiconductor element.
From these viewpoints, there have heretofore often been used a power semiconductor module as shown in FIG. 1 employing a heat radiation substrate. In FIG. 1, numeral 1 denotes a heat sink made of copper (Cu) and the like. On the heat sink 1, a first insulating plate Z.sub.1 made of aluminuim oxide Al.sub.2 O.sub.3 is joined through a solder layer 3a in order to insulate the heat sink from a thermal diffusion plate described below. On the insulating plate Z.sub.1, the thermal diffusion plate 4 is joined through a solder layer 3b. Further, on the thermal diffusion plate 4, second insulating plates Z.sub.2 made of Al.sub.2 O.sub.3 are joined through solder layers 3c in order to insulate the thermal diffusion plate 4 from a semiconductor elements to be mounted. On these insulating plates Z.sub.2, semiconductor elements 5 are joined through solder layers 3d, respectively. In this Figure, numeral 6 are each a joining layer formed between the insulating plate Z.sub.1 or Z.sub.2 and the solder layers.
However, a thermal radiation substrate used in conventional power semiconductor modules has a drawback that it is extremely complicated in construction as shown in FIG. 1. This is ascribable to the fact that Al.sub.2 O.sub.3 constituting the first and second insultation plates Z.sub.1 and Z.sub.2 is low in thermal conductivity (20 W/m..degree.C.) although high in dielectric strength (100 kV/cm), so that both functions, radiation and insulation, are required to be satisfied by employing both materials, Cu and Al.sub.2 O.sub.3 by themselves.
On the other hand, recently aluminium nitride (AlN) has attracted attention since it has excellent dielectric strength (140 to 170 kV/cm) and thermal conductivity (90 W/m..degree.C.) and tried to be joined to a copper (Cu) member to form a module substrate. AlN, however, is inferior in wettability with brazing materials hence it is difficult to obtain a sufficient joining strength even if joining of an AlN member to a Cu member are tried by use of silver brazing materials and the like.